The present invention relates generally to a method and an apparatus for dispensing low nanoliter volumes of a liquid while minimizing waste. More specifically, the present invention relates to a method for providing small, accurate liquid transfers. Such liquid transfers are useful, for example, in biomedical and biotechnology processes such as a high throughput screening process.
Industries, such as biotechnology and biomedical, use automated laboratory systems for producing and testing large numbers of chemical samples. These automated processes often require the manipulation of liquid samples, especially low volume liquid samples.
Current automated systems for high throughput screening can test over 100,000 samples a day. Many of the liquids used in these automated systems are expensive or available only in limited quantities. Therefore, current automated systems use small volumes of liquid to reduce costs and conserve liquid. However, when manipulating a small volume of liquid, the accuracy and precision of liquid transfers is critical to the reliability and reproducibility of the tests. Small errors or inaccuracies in dispensing a liquid can lead to test failure or the reporting of wrong results. Therefore, the need for accuracy and repeatability can restrict the ability of current systems to use very small volumes in screening. Further, current systems also often waste a substantial portion of the liquid before and after dispensing a small volume for use in the screening test. It is also important to minimize the volume of liquid solvents used, such as dimethyl sulfoxide (DMSO), as the solvent may affect the results of the testing.
Manual laboratory techniques for dispensing a small volume of a liquid are well known. For example, a laboratory technician may use a pipette to aspirate a quantity of a liquid, move the pipette to a sample well, and dispense a quantity of a liquid. However, the manual pipette operation is not capable of accurately dispensing volumes less than about several microliters. Such volumes are not only unacceptably large for automated processing, but the process of loading an entire sample plate is much too slow. Further, such a manual technique wastes a substantial quantity of liquid.
Current techniques also exist for automatically dispensing a limited number of samples in small volumes. However, the known techniques do not provide sufficient accuracy to dispense in the low nanoliter range or waste an unacceptable quantity of liquid. One such technique for dispensing nanoliter volumes requires aspirating a small quantity of a liquid into a syringe or pipette and then dispensing most or all of the aspirated liquid. However, due to the mechanical and structural limitations in operating the syringe or pipette, there results an unacceptably large variation in the actual amount of liquid aspirated. For example, many syringing systems suffer from mechanical backlash. Mechanical backlash results from the normal play between parts in a mechanical system. Accordingly, the mechanical components of the syringe system have one relationship during aspirating, but take on another relationship during dispensing. Even small variations in this mechanical relationship can significantly affect the quantity of liquid aspirated and dispense.
Syringe systems also suffer from variations in measurements depending on whether the syringe is aspirating or dispensing. For example, when aspirating, the syringe creates a low-pressure chamber for drawing in the liquid. The liquid may contain dissolved gases or small bubbles. Further, the low pressure chamber tends to distort the shape of chamber seals. However, when the syringe is dispensing, the dissolved gasses and seals are compressed. Accordingly, the syringe can aspirate a measured quantity of a liquid, but actually dispense a different quantity due to variations in liquid density and seal compression.
Also, after aspirating a liquid into a syringe, the tip of the syringe may retain a droplet. The size of the droplet on the tip will become significant in measuring the volume of liquid dispensed into a sample well as smaller volumes are dispensed.
Due to the limitations described above, current dispensing systems may not be able to accurately dispense in the low nanoliter volume range. For example, it may be desired to aspirate 50 nanoliters of a liquid. However, it would not be unusual, because of the structural and systematic limitations, that the actual quantity of liquid aspirated be anywhere in the range of 20 to 100 nanoliters. Since the quantity of liquid cannot be accurately aspirated, it follows that the amount dispensed is likewise inaccurate. Such large inaccuracies are unacceptable for proper test replication and reliability.
In an attempt to accommodate the structural and systematic problems described above, an alternate automated method of dispensing small volumes has been developed. Examples of systems and methods for dispensing liquids in small quantities are disclosed in U.S. Pat. Nos. 5,312,233; 5,763,278; 5,741,554; 5,785,926; 5,916,524; and 5,927,547.
In some alternate automated dispensing method, a relatively large volume of a liquid is aspirated into a syringe, the syringe primed, and then multiple small volumes dispensed. For example, 5,000 nanoliters (5 microliters) of a liquid can be aspirated into a syringe or pipette, a quantity of the liquid dispensed to prime the syringe, and then multiple low volume quantities dispensed until the syringe or pipette is empty. Accordingly, some of the operational and systematic inaccuracies are generally reduced by priming and by some spreading inaccuracies over multiple dispenses. In such a manner, each of the multiple volumes is somewhat more accurately dispensed then if a single dispense is made. However, this system suffers from wasting the liquid used to prime the syringe system. For example, if the process or screen needs only a single or a few dispenses in the low nanoliter volume range, then this conventional method of dispensing small volumes wastes large amounts of the liquid. Further, the low volume dispensers may still have unacceptably larger systematic inaccuracies.
Accordingly, there is a need for a method and an apparatus that can dispense low nanoliter volumes of liquids accurately and precisely while minimizing waste.
It is therefore an object of the present invention to provide an apparatus and method for accurately dispensing nanoliter volumes of a liquid. It is a separate object of the present invention to minimize waste of a dispensed liquid. The present invention alleviates to a great extent the disadvantages of the known methods and apparatus, for dispensing nanoliter volumes of liquids. Accordingly, the present invention improves the accuracy and precision in the dispensing of a low nanoliter volume of liquid, and minimizes wasted liquid.
Briefly, the present invention provides a method and apparatus for aspirating a volume of liquid into a syringe from a sample of the liquid. A substantial portion of the liquid in the syringe is subsequently dispensed from the syringe and returned to the sample. Accordingly, a small portion of the aspirated liquid is retained in the syringe. One or more low nanoliter volumes of the liquid retained in the syringe is then dispensed from the syringe to a container for further use or analysis. It will be understood, therefore, that the volume of the originally aspirated liquid is substantially larger than the volume of liquid dispensed for further use or analysis. The invention also includes a method and apparatus for aspirating and dispensing a plurality of liquids by using a plurality of syringes, and also can be used to dispense nanoliter volumes of liquids into microplates.
The method and apparatus for dispensing low nanoliter volumes involves accurately dispensing one or more samples with minimal waste. The method also substantially reduces systematic error introduced by mechanical backlash, for example. Further, waste is substantially reduced as compared to known methods.
These and other features and advantages of the present invention will be appreciated from review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout.